Silicon based thin film solar cell

ABSTRACT

According to the present invention, sufficient light trapping effect can be exhibited and series resistance can be kept small, by sequentially forming a silicon based low refractive index layer and a thin silicon based interface layer on a backside of a photoelectric conversion layer observed from a light incident side, and as a result a silicon based thin film solar cell may be provided efficiently and at low cost.

This application is a national phase of PCT applicationPCT/JP2004/010248 filed on Jul. 12, 2004, claiming priority to JapaneseApplication Nos. 2003-279491 filed on Jul. 24, 2003 and 2003-358362filed Oct. 17, 2003, the entirety of which are incorporated byreference.

TECHNICAL FIELD

The present invention relates to a silicon based thin film solar cell,and in particular to a thin film solar cell enabling demonstration oflight trapping effect by disposing a layer having a smaller refractiveindex than a refractive index of the photoelectric conversion layer on abackside of a photoelectric conversion layer observed from a lightincident side.

BACKGROUND ART

In recent years, various thin film solar cells have come into use, inaddition to conventional amorphous thin film solar cells, crystallinethin film solar cells are also being developed, and moreover hybrid thinfilm solar cells obtained by laminating these solar cells together arealso put in practical use.

Thin film solar cells in general comprise a first electrode, one or moresemiconductor thin film photoelectric conversion units, and a secondelectrode laminated in an order on a substrate. And one photoelectricconversion unit comprises an i type layer sandwiched by a p type layerand an n type layer.

The i type layer is substantially an intrinsic semiconductor layer,occupies a large percentage of a thickness of the photoelectricconversion unit, and then photoelectric conversion effect is generatedmainly within this i type layer. For this reason, this i type layer isusually referred to as an i type photoelectric conversion layer, orsimply as a photoelectric conversion layer. The photoelectric conversionlayer is not limited to an intrinsic semiconductor layer, but may be alayer obtained by being doped, within a range in which loss of lightabsorbed with doped impurity does not cause problems, into a p type oran n type in a very small quantity range. Although a thickerphotoelectric conversion layer is more preferable for light absorption,a layer thicker than necessary may cause results of increasing cost forfilm-forming and time for production.

On the other hand, conductivity type layers of a p type or an n typeexhibit function to generate a diffusion potential in a photoelectricconversion unit, a magnitude of this diffusion potential influences avalue of an open circuit voltage as one of important characteristics ofa thin film solar cell. However, these conductivity type layers areinert layers not directly contributing to photoelectric conversion, andthus light absorbed with impurity doped in the conductivity type layergives loss not contributing to generation of electric power. Therefore,the conductivity type layers of the p type and the n type are preferablymaintained for a smallest possible thickness within a range forgeneration of a sufficient diffusion potential.

Here, in the above-mentioned a pin (nip) type photoelectric conversionunit or a thin film solar cell, when a photoelectric conversion layeroccupying a principal portion is amorphous, it is called an amorphousunit or an amorphous thin film solar cell, and when a photoelectricconversion layer is crystalline, it is called a crystalline unit or acrystalline thin film solar cell, regardless of whether conductivitytype layers of p type and n type included therein are amorphous orcrystalline.

As a method of improving conversion efficiency of a thin film solarcell, a method of laminating two or more photoelectric conversion unitsto obtain a tandem unit may be mentioned. In this method, a front unitcomprising a photoelectric conversion layer having a larger band gap isdisposed on a light incident side of a thin film solar cell, and a backunit comprising a photoelectric conversion layer having a smaller bandgap is disposed in an order on a back side of the front unit, and thisconfiguration thereby enables photoelectric conversion over a large waverange of an incident light, and realizes improvement in conversionefficiency as a whole solar cell. Among such tandem solar cells,especially a solar cell having an amorphous photoelectric conversionunit and a crystalline photoelectric conversion unit laminated togetheris referred to as a hybrid thin film solar cell.

For example, in a longer wavelength side, an i type amorphous siliconeexhibits photoelectric conversion function in wavelength of a light upto about 800 nm, while an i type crystalline silicone can exhibitphotoelectric conversion function with a light of longer wavelength ofabout 1100 nm. However, although a light absorption even with asufficient thickness of about not more than 0.3 micrometers can berealized in an amorphous silicone photoelectric conversion layer havinga larger light absorption coefficient, in a crystalline siliconephotoelectric conversion layer having a smaller light absorptioncoefficient, in order to fully absorb light of a longer wavelength, thelayer preferably has a thickness of about 1.5 to 3 micrometers. That is,usually a crystalline photoelectric conversion layer preferably has athickness of about 5 to 10 times as large as a thickness of an amorphousphotoelectric conversion layer.

In a monolayer amorphous silicon thin film solar cell, and also in theabove-mentioned hybrid thin film solar cell, a thickness of aphotoelectric conversion layer is desirably maintained as small aspossible, from a viewpoint of improvement in productivity, that is, costreduction. For this reason, generally used is a structure using what iscalled light trapping effect in which a disposition of a layer having arefractive index smaller than a refractive index of a photoelectricconversion layer, on a backside of the photoelectric conversion layerobserved from a light incident side enables effective reflection oflight of a particular wavelength. A disposition on a backside of aphotoelectric conversion layer observed from a light incident side meansa disposition contacting to the photoelectric conversion layer on a sideof a back face, or a disposition on a side of a back face in a state ofsandwiching an other layer disposed on a back face of the photoelectricconversion layer.

Japanese Patent Laid-Open No. 02-73672 official report discloses astructure of a solar cell in which a translucent first electrode, anamorphous silicon semiconductor thin film (hereinafter referred to assimply semiconductor thin film), a zinc oxide film having a thickness ofless than 1200 angstroms, a non-translucent second electrode (metalelectrode) are laminated in this order from a light incident side. Thezinc oxide film has a function for preventing a silicide formed in aninterface between the semiconductor thin film and the metal electrodeincrease absorption loss. Since refractive index difference existsbetween the zinc oxide film and the semiconductor thin film, a thicknessof the zinc oxide film limited to a range of less than 1200 angstromsand preferably to a range of 300 angstroms to 900 angstroms has aneffect of improving reflectance in an interface of the semiconductorthin film/the zinc oxide film. For this reason, a short-circuit currentdensity of the solar cell and consequently a conversion efficiencyimproves. However, since the zinc oxide film is formed by a technique ofsputtering, spraying, etc., different facilities from that forsemiconductor thin film formed in general by plasma CVD methods etc. arerequired, leading to occurrence of problems of facility cost rise andlonger production tact. Furthermore, there may occur problems thatespecially use of sputtering method in formation of the zinc oxide filmmay cause performance reduction by sputter damage to a groundsemiconductor thin film. According to examples, the above-mentionedsemiconductor thin film consists of a P type a-SiC:H film, a non dopeda-Si:H film, and an n type a-Si:H film. In this case in order togenerate sufficient diffusion potential in a non doped a-Si:H film, athickness of an n type a-Si:H film requires 150 angstroms to 300angstroms in general, which will not permit ignoring absorption loss oflight passing through the n type a-Si:H film.

Japanese Patent Laid-Open No. 4-167473 official report discloses astructure, in a sequential order from light incident side, of atransparent electrode/one electric conductive type amorphoussemiconductor layer/an intrinsic amorphous semiconductor layer/anamorphous silicon oxynitride or amorphous silicon oxide (hereinafterreferred as a-SiON or a-SiO)/a metal oxide layer/a high reflective metallayer/a substrate. However, this a-SiON (a-SiO) layer is formed forprevention of increase in absorption loss by reduction of the metaloxide layer that may be obtained when forming the amorphoussemiconductor layer on the metal oxide layer, and no description isdisclosed that light trapping may be performed using refractive indexdifference between the a-SiON (a-SiO) layer and the intrinsic amorphoussemiconductor layer. Specifically, in Examples, a thickness of a-SiON(a-SiO) layer set thin as 200 angstroms does not permit expectation ofsufficient light trapping effect.

Japanese Patent Laid-Open No. 6-267868 official report discloses amethod for forming a film of a-SiO including microcrystalline phase ofsilicon characterized by being based on decomposition of a source gashaving not more than 0.6 of a value of CO₂/(SiH₄+CO₂). The officialreport describes that this film represents a high photoconductivity notless than 10⁻⁶ S/cm, and a low absorption coefficient, and is suitablefor a window layer of amorphous silicon based solar cells. However, thisofficial report fails to describe about a refractive index of theobtained film, and fails to describe that the film can be disposed on abackside of a photoelectric conversion layer of the solar cell observedfrom a light incident side. The present inventors carried outinvestigation for application of a silicon oxide layer by a highfrequency plasma CVD method for an n type layer of pin type siliconbased thin film solar cell using SiH₄, CO₂, H₂, and PH₃ as reactive gas,based on teachings obtained by the documentary materials. As a result,it was found our that using a technique of disposing a silicon oxidelayer on a backside of a photoelectric conversion layer, and of settinga ratio of CO₂/SiH₄ larger, light trapping effect was exhibited and ashort-circuit current of the solar cell was increased when increasing anamount of oxygen in the layer and a difference of refractive index withthe photoelectric conversion layer. However, only simple use of thesilicon oxide as an n type layer increased a series resistance of thesolar cell, leading to a problem of reduction of conversion efficiency.This is considered to originate in a contact resistance between siliconoxide and a metal oxide layers, such as ZnO as a part of a backelectrode.

Thus, conventional technique cannot solve a problem of series resistanceof solar cells that is believed to be caused by a contact resistancegenerated between a silicon based low-refractive index layer representedby silicon oxides, and a back electrode.

DISCLOSURE OF INVENTION

Taking the above situations into consideration, by disposing a layerhaving a lower refractive index compared with that of a photoelectricconversion layer, on a backside of the photoelectric conversion layerobserved from a light incident side, without using different facilitiesfrom those for formation of the photoelectric conversion layer, thepresent invention aims at providing a silicon based thin film solarcell, efficiently and at a low cost, that can exhibit sufficient lighttrapping effect and can keep a series resistance of the solar cellsmaller even if a layer having a low refractive index is disposed.

A silicon based thin film solar cell by the present invention ischaracterized in that a silicon based low refractive index layer and asilicon based interface layer are disposed in this order on a backsideof a photoelectric conversion layer observed from a light incident side.

The silicon based low refractive index layer has a function to generatea diffusion potential in the photoelectric conversion layer, which is alayer doped with impurity to give a p type or an n type. In order toeffectively reflect light on a surface thereof to the photoelectricconversion layer side, and to keep absorption loss of light in the layeras small as possible, the silicon based low refractive index layerpreferably has a refractive index of not more than 2.5 at a wavelengthof 600 nm, and has a thickness of not less than 300 angstroms.

A silicon based low refractive index layer is an alloy layer comprisingsilicon and elements, such as oxygen, typically a silicon oxides,preferably a ratio of a most abundantly existing constituent element,excluding silicon, in the layer is not less than 25 atom %, and thelayer is preferably formed by methods, such as a high frequency plasmaCVD that are same kind as methods for a photoelectric conversion layer.The silicon based low refractive index layer preferably includescrystalline silicon components in the layer, in order to reduce aresistance in a thickness direction of the layer itself.

A silicon based interface layer is a conductivity type layer havingsilicon as a principal component. Since the silicon based interfacelayer does not need to contribute to generation of a diffusion potentialin the photoelectric conversion layer, it preferably has a thickness ofnot more than 150 angstroms, and more preferably has a thickness of notmore than 100 angstroms in order to keep light absorption loss in thelayer as small as possible. Furthermore, in order to keep a contactresistance with back electrode small, it preferably comprises acrystalline silicon component in the layer.

In order to solve problems of increase in a series resistance of a solarcell caused by disposition of a silicon based low refractive index layerin a backside of a photoelectric conversion layer, the present inventorswholeheartedly investigated structures for optimal solar cells. As aresult, it was found out that by disposing a thin silicon basedinterface layer in a backside of a silicon based low refractive indexlayer, a contact resistance to a back electrode layer comprising metaloxide layer disposed in a backside thereof was improved, and thereby aseries resistance of the solar cell became smaller and conversionefficiency was improved.

In the present invention, a silicon based interface layer disposedbetween a silicon based low refractive index layer and a back electrodelayer has small contact resistances with either of the silicon based lowrefractive index layer and the back electrode layer, and as a result asmall series resistance of the solar cell is believed to be realized.Especially, as shown in FIG. 1, when silicon oxide is used as a siliconbased low refractive index layer, and an amount of oxygen in the layeris increased to lower a refractive index to not more than 2.5, it isdifficult to lower a contact resistance between the silicon based lowrefractive index layer and the back electrode layer. However, such aproblem is solved by inserting a silicon based interface layer.Therefore, this technique enables design of the silicon based lowrefractive index layer to an optimal thickness and an optimal refractiveindex for light trapping. Furthermore, since simple change offilm-forming conditions permits adjustment of a refractive index of thesilicon based low refractive index layer, increase in light trappingeffect by more delicate optical designs can also be expected, such asperiodic variation of a refractive index in a thickness direction.

Herein a description will be given about a silicon based thin film solarcell as an embodiment of the invention referring to FIG. 2.

A transparent electrode layer 2 is formed on a translucent board 1. Asthe translucent board 1, a tabular member and a sheet shaped membercomprising a glass, a transparent resin, etc. are used. The transparentelectrode layer 2 preferably comprises conductive metal oxides, such asSnO₂ and ZnO, and preferably formed using methods, such as CVD,sputtering, and vapor deposition. The transparent electrode layer 2preferably has minute unevenness formed on a surface thereof, andpreferably has an effect of increasing dispersion of incident light. Anamorphous photoelectric conversion unit 3 is formed on the transparentelectrode layer 2. The amorphous photoelectric conversion unit 3comprises an amorphous p type silicon carbide layer 3 p, non dopedamorphous i type silicon photoelectric conversion layer 3 i, and an ntype silicon based interface layers 3 n. A crystalline photoelectricconversion unit 4 is formed on the amorphous photoelectric conversionunit 3. A high frequency plasma CVD method is suitable for formation ofthe amorphous photoelectric conversion unit 3 and the crystallinephotoelectric conversion unit 4 (both of the units are simply,hereinafter, collectively referred to as photoelectric conversion unit).As formation conditions for the photoelectric conversion unit,preferably used are conditions of: substrate temperature 100 to 300degrees C.; pressure 30 to 1500 Pa; and high frequency power density0.01 to 0.5 W/cm². As a source gas used for photoelectric conversionunit formation, silicon including gases, such as SiH₄ and Si₂H₆ etc. ora mixed gas thereof with H₂ are used. As dopant gas for forming a p typeor an n type layer in the photoelectric conversion unit, B₂H₆ or PH₃etc. is preferably used.

The crystalline photoelectric conversion unit 4 comprises a crystallinep type silicon layer 4 p; a crystalline i type silicon photoelectricconversion layer 4 i; an n type silicon based low refractive index layer4 on; and an n type silicon based interface layer 4 n. As a n typesilicon based low refractive index layer 4 on, silicon oxide istypically used, and in the case a mixed gas of SiH₄, H₂, CO₂, and PH₃ issuitable for a source gas to be used. The silicon based low refractiveindex layer 4 on may or may not comprise crystalline silicon components.A refractive index at a wavelength of 600 nm of the silicon based lowrefractive index layer 4 on is preferably not more than 2.5. Apercentage of a most abundantly existing constituent element exceptsilicon in a layer in the silicon based low refractive index layer 4 onis preferably not less than 25 atomic %. A thickness of the siliconbased low refractive index layer 4 on is preferably not less than 300angstroms, and more preferably 500 angstroms to 900 angstroms. Whensilicon oxide is used as the silicon based low refractive index layer 4on, in order to realize a percentage of oxygen occupied in the layer, ora refractive index thereof, a gas ratio of CO₂/SiH₄ is approximately 2to 10. The silicon based low refractive index layer 4 on may have afixed refractive index in a thickness direction, or may have refractiveindexes varying in a thickness direction. Furthermore, it may haverefractive indexes periodically variable. FIG. 2 shows a structure thatthe n type silicon based low refractive index layer 4 on is disposed,contacting the crystalline i type silicon photoelectric conversion layer4 i, on a backside of the crystalline i type silicon photoelectricconversion layer 4 i observed from a light incident side. Other layers,such as an n type silicon layer, may be disposed in a state sandwichedbetween the crystalline i type silicon photoelectric conversion layer 4i and the n type silicon based low refractive index layer 4 on. And thesilicon based low refractive index layer 4 on may be a layer, instead ofsilicon oxide, including any one or more elements of nitrogen, carbon,and oxygen in addition to silicon, such as silicon nitride, siliconcarbide, silicon oxy-nitride, silicon oxy-carbide, etc.

An n type silicon based interface layer 4 n is formed on the n typesilicon based low refractive index layer 4 on. Crystalline silicon ismainly used for the n type silicon based interface layer 4 n. The n typesilicon based interface layer 4 n is used in order to improve a contactresistance between the n type silicon based low refractive index layer 4on and a back electrode 5, and thus preferably has a thickness as smallas possible in order to minimize light absorption loss in this layer.Specifically, the thickness is not more than 150 angstroms, and morepreferably not more than 100 angstroms. Furthermore, as the n typesilicon based interface layer 4 n, a layer having an electricconductivity of about 1 to 10² S/cm is used. The n type silicon basedinterface layer 4 n may include any one or more elements of oxygen,carbon, and nitrogen in a range not increasing a contact resistance withthe back electrode 5.

The back electrode 5 is formed on the n type silicon based interfacelayer 4 n. The back electrode 5 consists of a transparent reflectinglayer 5 t, and a back reflecting layer 5 m. Metal oxides, such as ZnOand ITO, are preferably used for the transparent reflecting layer 5 t,and Ag, Al, or alloys thereof are preferably used for the backreflecting layer 5 m. In formation of the back electrode 5, methods suchas sputtering and vapor deposition, are preferably used. Although FIG. 2indicates a structure of a hybrid thin film solar cell, a number of thephotoelectric conversion units 4 needs not necessarily to be two, and itmay be amorphous or crystalline and may have a solar cell structure withmonolayer or three or more layer-type. Furthermore FIG. 2 shows astructure in which a photoelectric conversion layer, a silicon based lowrefractive index layer, and an n type silicon based interface layer aredisposed on a translucent board in this order, or it may have what iscalled a reverse type structure in which an n type silicon basedinterface layer, a silicon based low refractive index layer, and aphotoelectric conversion layer are deposited in this order on aconductive boards of such as metal, or an insulated substrate. Thepresent invention corresponds to a patent application concerningachievement of sponsored research by the government. (Japanese, NewEnergy and Industrial Technology Development Organization in Heisei 15fiscal year “Photovoltaic power generation technical researchdevelopment commission enterprise”, Article 30 of the Law concerningTemporary Measures for Industrial Revitalization is applied to thepresent application.)

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a figure showing a relationship of an amount of oxygen in alayer and a refractive index of silicon based low refractive indexlayer;

FIG. 2 is a schematic sectional view of a thin film solar cellcomprising a silicon based low refractive index layer by the presentinvention;

FIG. 3 is a schematic sectional view of hybrid thin film solar cellsproduced in each Example and Comparative Example;

FIG. 4 is a figure showing a reflection spectrum in which light wasentered and was measured from a surface exposed by etching removal of aback electrode of solar cells produced by Example 1 and ComparativeExample 1;

FIG. 5 is a figure showing a relationship between a refractive index ofa silicon based low refractive index layer, and conversion efficiency ofa hybrid thin film solar cell;

FIG. 6 is a figure showing a relationship between a thickness of siliconbased low refractive index layer, and conversion efficiency of a hybridthin film solar cell; and

FIG. 7 is an expanded sectional view by transmission electron microscope(TEM) photograph of a silicon based thin film solar cell of the presentinvention obtained in Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

Descriptions will, hereinafter, be given for Examples 1, 2, and 3 as asilicon based thin film solar cell by the present invention, comparingwith Comparative Examples 1 and 2 with reference to FIG. 3.

Example 1

FIG. 3 is a sectional view showing schematically a hybrid thin filmsolar cell produced in each Example and each Comparative Example.

First, a transparent electrode layer 2 consisting of SnO₂ and having aminute uneven structure on a surface thereof was formed by a heat CVDmethod on a principal surface of a translucent board 1 consisting a blueplate glass of a thickness of 0.7 mm.

Next, in order to form an amorphous photoelectric conversion unit 3, thetranslucent board 1 having a transparent electrode layer 2 formedthereon was introduced in a high frequency plasma CVD device. After theequipment was heated up to a predetermined temperature, an amorphous ptype silicon carbide layer 3 p with a thickness of 160 angstroms, a nondoped amorphous i type silicon photoelectric conversion layer 3 i with athickness of 3000 angstroms, and an n type silicon layer 3 n with athickness of 300 angstroms were sequentially laminated.

Furthermore, in order to form a crystalline photoelectric conversionunit 4, using a plasma CVD device, a p type crystalline silicon layer 4p with a thickness of 150 angstroms, a crystalline i type siliconphotoelectric conversion layer 4 i with a thickness of 1.4 micrometers,an n type silicon based low refractive index layer 4 on with a thicknessof 600 angstroms, and an n type crystalline silicon based interfacelayer 4 n with a thickness of 50 angstroms to 70 angstroms weresequentially laminated. Film-forming conditions of the n type siliconbased low refractive index layer 4 on in the case are shown below: adistance between substrate film-forming side-electrode of 10 to 15 mm; apressure of 350 to 1300 Pa; a high frequency power density of 0.1 to0.13 W/cm²; and flow rates of SiH₄/CO₂/PH₃/H₂ of 15/120/0.5/9000 sccm,respectively. A refractive index of an n type silicon based lowrefractive index layer deposited by a thickness of 2500 angstroms on aglass using identical film-forming conditions was measured for byspectrum ellipsometry to give 1.9 at a wavelength of 600 nm. On theother hand, film-forming conditions of then type silicon based interfacelayer 4 n are shown below: a distance between substrate film-formingside-electrode of 10 to 15 mm; a pressure of 350 to 1300 Pa; a highfrequency power density of 0.11 W/cm²; and flow rates of SiH₄/PH₃/H₂ of20/0.5/2500 sccm respectively. An electric conductivity of an n typesilicon based interface layer deposited with a thickness of 2500angstroms on a glass on identical film-forming conditions gave 12 S/cm.

Then, as back electrodes 5, a transparent reflecting layer consisting ofZnO with a thickness of 300 angstroms (not shown) and a back reflectinglayer consisting of Ag with a thickness of 2000 angstroms (not shown)were formed using a DC sputtering method.

Furthermore, in order to isolate the amorphous photoelectric conversionunit 3, the crystalline photoelectric conversion unit 4, and the backelectrode 5 in a shape of an island, while leaving the transparentelectrode layer 2, two or more of back electrode layer isolation grooves5 a were formed by irradiating a YAG second harmonics pulsed laser tothe translucent board 1. Although not shown, two or more back electrodeisolation grooves perpendicularly intersecting to the back electrodelayer isolation groove 5 a were also formed to give island-like isolatedareas. Furthermore, on outside of the island-like isolated area adjacentto the back electrode layer isolation groove 5 a a back electrode layerisolation groove was further formed, and then solder was permeated to aninside thereof to form a contact area 6 with respect to the transparentelectrode layer 2. Thus, a hybrid thin film solar cell was produced.This hybrid thin film solar cell has an effective area of 1 cm². InExample 1, totally 25 of the solar cells were produced on one substrate.

Pseudo-solar light having a spectrum distribution AM 1.5 and an energydensity 100 mW/cm² was irradiated to the hybrid thin film solar cellproduced in Example 1 under a condition of temperature of measurementatmosphere and solar cell as 25±1 degrees C. A voltage and an electriccurrent between a positive electrode probe 7 in contact with thetransparent electrode layer 2 through the contact area 6, and a negativeelectrode probe 8 in contact with the back electrode 5 were measured toobtain an output characteristic of the thin film solar cell. Table 1shows an average performance of 25 hybrid thin film solar cells producedin Example 1.

A part of the solar cell was dipped in a nitric acid aqueous solution,and etching removal of the back electrode 5 was carried out to exposethe n type silicon based interface layer 4 n. In this state, light wasirradiated from a side of the n type silicon based interface layer 4 n,and a reflection spectrum was measured for. FIG. 4 shows the reflectionspectrum. Next, then type silicon based interface layer 4 n was removedby a reactive ion etching (RIE) method to expose the n type siliconbased low refractive index layer 4 on. A refractive index of thissilicon based low refractive index layer measured by a spectrumellipsometry gave 1.93 at a wavelength of 600 nm. Moreover, an amount ofoxygen in the silicon based low refractive index layer measured by anX-ray photoelectron spectroscopy (XPS) gave 48 atomic %.

Example 2

In Example 2, an almost similar process as in Example 1 was carried outexcept for having varied a refractive index at a wavelength of 600 nm ina range of 1.65 to 2.65 by varying only film-forming conditions of an ntype silicon based low refractive index layer 4 on. FIG. 5 shows arelationship between refractive indexes of the silicon based lowrefractive index layer, and conversion efficiency of the hybrid thinfilm solar cell.

Example 3

In Example 3, an almost similar process as in Example 1 was carried outexcept for having varied a thickness of an n type silicon based lowrefractive index layer 4 on in a range of 100 angstroms to 1000angstroms. FIG. 6 shows a relationship between a thickness of thesilicon based low refractive index layer and obtained conversionefficiency of the hybrid thin film solar cell.

Comparative Example 1

In Comparative Example 1, only following points differed from inExample 1. Instead of laminating sequentially an n type silicon basedlow refractive index layer 4 on and an n type crystalline silicon basedinterface layer 4 n, an n type crystalline silicon layer with athickness of 150 angstroms and a ZnO layer with a thickness of 600angstroms were laminated sequentially. Film-forming of ZnO layer wasperformed by a DC sputtering method. A ZnO layer deposited with athickness of 2500 angstroms on a glass under an identical film-formingcondition was measured for a refractive index by spectrum ellipsometryto give 1.9 at a wavelength of 600 nm. Table 1 shows an averageperformance of 25 hybrid thin film solar cells produced in ComparativeExample 1. A part of the solar cells produced in Comparative Example 1was dipped in a nitric acid aqueous solution, and etching removal of theback electrode 5 was carried out to expose the n type silicon basedinterface layer 4 n. Light was irradiated from the n type crystallinesilicon layer side in this state to obtain a reflection spectrum. FIG. 4shows the reflection spectrum.

TABLE 1 Average solar cell performance Configuration of a crystallineOpen Short-circuit photoelectric conversion unit circuit currentConversion (only shown layers above p type voltage (mA) Curvilinearefficiency crystalline Si layer) (V) (V) (mA) factor (%) Example 1 itype crystalline Si layer/n 1.375 11.86 0.725 11.82 type silicon basedlow refractive index layer/n type silicon based interface layer/ ZnOlayer Comparative i type crystalline Si layer/n 1.374 11.39 0.739 11.57Example 1 type silicon layer/ZnO layer Comparative i type crystalline Silayer/n 1.378 11.74 0.696 11.26 Example 2 type silicon based lowrefractive index layer/ZnO layer

Comparative Example 2

In Comparative Example 2, a similar process as in Example 1 was carriedout except for a point of having omitted formation of an n type siliconbased interface layer 4 n on an n type silicon based low refractiveindex layer 4 on. Table 1 shows an average performance of 25 integratedhybrid thin film solar cells produced in Comparative Example 2.

Comparison between Example 1 and Comparative Example 1 shows that inExample 1 a short-circuit current is improved not less than 4% ascompared to that in Comparative Example 1. The reason is shown below. InExample 1, a great portion of a light reaching on a backside of thecrystalline i type silicon photoelectric conversion layer 4 i wasreflected into a side of the crystalline i type silicon photoelectricconversion layer 4 i, at an interface between the crystalline i typesilicon photoelectric conversion layer 4 i and the n type silicon basedlow refractive index layer 4 on, and consequently a percentage of lightpassing through the n type crystalline silicon based interface layer 4 nhaving a large light absorption loss decreased. On the other hand, inComparative Example 1, the n type crystalline silicon layer and the ZnOlayer are sequentially laminated on a backside of the crystalline i typesilicon photoelectric conversion layer 4 i, and therefore, a percentageof light passing through the n type crystalline silicon layer having alarge light absorption loss increased. And furthermore, in Example 1,damage given to a ground crystalline silicon layer at the time ofsputtering of ZnO layer possibly formed in the process of ComparativeExample 1 was prevented.

Next, comparison between Example 1 and Comparative Example 2 shows thata fill factor in Example 1 is improved about 5% as compared to that inComparative Example 2. This is based on a reason that in Example 1,disposition by insertion of the n type crystalline silicon basedinterface layer 4 n between the n type silicon based low refractiveindex layer 4 on and the transparent reflecting layer 5 t improves aseries resistance of the solar cell.

A test result of a reflection spectrum obtained by measuring withirradiated light from a light incident side and an opposite direction atthe time of solar cell characteristics measurement shown in FIG. 4 showsthat etching removal of the back electrode 5 enables detection ofwhether a silicon based low refractive index layer 4 on having a smallerrefractive index is disposed on a backside of a crystalline i typesilicon photoelectric conversion layer 4 i. Results of Example 2 in FIG.5 shows that a refractive index of the silicon based low refractiveindex layer has an optimal value, which is preferably not more than 2.5.

FIG. 1 shows that this condition corresponds to a value of not less than25 atomic % of an amount of oxygen in the layer. This is based on areason that a refractive index exceeding 2.5 makes a refractive indexdifference with an adjoining crystalline i type silicon photoelectricconversion layer smaller, which reduces light trapping effect. Resultsof Example 3 shown in FIG. 6 shows that a thickness of the silicon basedlow refractive index layer has an optimal value, which is preferably notless than 300 angstroms.

According to the present invention from the above description,sufficient light trapping effect at low cost can be exhibited bydisposing a layer having a lower refractive index compared to that of aphotoelectric conversion layer, on a backside of the photoelectricconversion layer observed from a light incident side, without usingdifferent facilities from those for formation of the photoelectricconversion layer. Furthermore, by disposing a thin silicon basedinterface layer on a backside of a silicon based low refractive indexlayer, a series resistance of a solar cell can be kept small. As aresult, a silicon based thin film solar cell can be provided efficientlyand at low cost.

INDUSTRIAL APPLICABILITY

According to the present invention, sufficient light trapping effect atlow cost can be exhibited by disposing a layer having a lower refractiveindex compared with that of a photoelectric conversion layer, on abackside of the photoelectric conversion layer observed from a lightincident side, without using different facilities from those forformation of the photoelectric conversion layer. Furthermore, bydisposing a thin silicon based interface layer on a backside of asilicon based low refractive index layer, a series resistance of a solarcell can be kept small. As a result, a silicon based thin film solarcell can be provided efficiently and at low cost.

1. A silicon based thin film solar cell, wherein a conductive typesilicon based low refractive index layer, a silicon based interfacelayer, and a back electrode are disposed and contact one another in thisorder on a backside of a photoelectric conversion layer observed from alight incident side, wherein the silicon based interface layer comprisesa crystalline silicon component in the layer, wherein the silicon basedlow refractive index layer comprises a crystalline silicon component inthe layer, wherein a most abundantly existing constituent element,excluding silicon, in the silicon based low refractive index layer isoxygen, which is present in an amount not less than 25 atomic %, andwherein the silicon based low refractive index layer has a thickness ofnot less than 300 angstroms.
 2. The silicon based thin film solar cellaccording to claim 1, wherein the silicon based low refractive indexlayer has a refractive index not more than 2.5 at a wavelength of 600nm.
 3. The silicon based thin film solar cell according to claim 1,wherein the silicon based interface layer has a thickness not more than150 angstroms.
 4. The silicon based thin film solar cell according toclaim 1, wherein the silicon based low refractive index layer andsilicon based interface layer includes the same conductivity type.